Frequency references provided by oscillators are required in every clocked electronic system, including communication circuits, microprocessors, and signal processing circuits. Oscillators frequently consist of high performance piezoelectric crystals, such as quartz oscillators. The advantages of quartz oscillators are their stable operating frequency and high quality factor. However, the disadvantages of quartz oscillators are their relatively large size and unsuitability for high integration with electronic circuitry (e.g., CMOS circuits).
Based on these limitations of conventional oscillators, there is a strong interest in the development of fully integrated silicon oscillators. Integration is important not only for reduced size but also reduced power consumption. It is possible to realize an integrated silicon oscillator using the mechanical properties of silicon devices. For example, silicon microelectromechanical (MEMs) resonators can provide small form factor, ease of integration with conventional semiconductor fabrication techniques and high f·Q products. Accordingly, MEMs resonators are considered a desirable alternative to quartz resonators in real-time and other clock applications.
One category of MEMs resonators includes lateral-mode piezoelectric resonators, such as thin-film piezoelectric-on-silicon (TPoS) resonators. These resonators utilize piezoelectric transduction to achieve electromechanical coupling using the transverse piezoelectric coefficient e31 of a planar piezoelectric layer, such as aluminum nitride (AlN). These types of resonators are disclosed in U.S. Pat. No. 7,939,990 to Wang et al., entitled “Thin-Film Bulk Acoustic Resonators Having Perforated Bodies That Provide Reduced Susceptibility to Process-Induced Lateral Dimension Variations,” and in U.S. Pat. No. 7,888,843 to Ayazi et al., entitled “Thin-Film Piezoelectric-on-Insulator Resonators Having Perforated Resonator Bodies Therein,” the disclosures of which are hereby incorporated herein by reference. An example of a high frequency vertical bulk acoustic resonator with a relatively large transduction area that utilizes capacitive coupling is disclosed in U.S. Pat. No. 7,176,770 to Ayazi et al., entitled “Capacitive Vertical Silicon Bulk Acoustic Resonator,” the disclosure of which is hereby incorporated herein by reference.
Additional examples of MEMs resonators, which provide electromechanical coupling using the transverse piezoelectric coefficient e31 of a planar piezoelectric layer and include active frequency tuning, are disclosed in U.S. Pat. Nos. 7,639,105 and 7,843,284 to Ayazi et al., entitled “Lithographically-Defined Multi-Standard Multi-Frequency High-Q Tunable Micromechanical Resonators,” and in U.S. Pat. No. 7,924,119 to Ayazi et al., entitled Micromechanical Bulk Acoustic Mode Resonators Having Interdigitated Electrodes and Multiple Pairs of Anchor Supports,” and in U.S. Pat. No. 7,800,282 to Ayazi et al., entitled Single-Resonator Dual-Frequency Lateral-Extension Mode Piezoelectric Oscillators, and Operating Methods Thereof,” the disclosures of which are hereby incorporated herein by reference. MEMs resonators, which support actuation and sensing through a longitudinal piezoelectric effect, are also disclosed in U.S. Pat. No. 8,519,598 to Ayazi et al., the disclosure of which is hereby incorporated herein by reference.
Efforts have also been made to utilize these MEMs resonators in temperature-compensated and oven-controlled quartz crystal oscillator (TCXO and OCXO) markets. However, given the relatively large temperature sensitivity of silicon resonators compared to their quartz crystal counterparts, realization of high frequency stabilities over a large temperature range typically requires highly accurate and continuous temperature monitoring of the resonator to thereby apply proper pulling and/or temperature control, which can compensate for temperature-induced frequency drifts. Unfortunately, accurate CMOS temperature sensing solutions typically impose excessive power consumption and complexity. And, because they are often physically separate and thermally isolated from the micromechanical frequency reference, such CMOS-based techniques are prone to offset and delay in temperature monitoring of the resonator, which can result in frequency drifts exceeding instability tolerances.
Accordingly, device-level temperature sensing techniques have been developed. For example, dual-mode quartz crystal and MEMs resonant temperature sensors have been designed and implemented using a higher tone of the main resonance mode along with the first tone to generate a small beat frequency with large temperature sensitivity. One example a of dual-mode MEMs-based oscillator is disclosed in an article by M. J Dalal et al., entitled “Simultaneous Dual-Mode Excitation of Piezo-On-Silicon Micromechanical Oscillator For Self-Temperature Sensing,” IEEE MEMS 2011, Cancun, Mexico, January 2011, pp. 489-492. However, these techniques may be limited by the relatively small TCF variation of different tones of a resonance mode. Moreover, similar particle polarization and/or common electrical transduction ports used for excitation and sensing of both modes may result in destructive mechanical and/or electrical interference when a device operates in two oscillation loops simultaneously.